Shallow trench isolation (STI) regions are formed in integrated circuits to isolate devices. In the formation process of conventional STI regions, openings are first formed in semiconductor substrates. Silicon oxide liners are formed in the openings, followed by a gap filling process, wherein the remaining portions of the openings are filled with a dielectric material. A chemical mechanical polish (CMP) is then performed to remove excess portions of the dielectric material. The portions of the dielectric material and the silicon oxide liner that are left in the openings thus form the STI regions.
High-density plasma (HDP) chemical vapor deposition (CVD) was typically used for the gap-filling in the formation of the STI regions. In recent years, high-aspect ratio process (HARP) was sometimes used to replace the HDP for the gap filling process. The STI regions formed using the HARP is not as dense as the STI regions formed using the HDP. To prevent the STI regions formed using the HARP from being etched excessively in subsequent wet etching processes, the respective wafer is annealed at a high temperature for a long time to condense the STI regions.
The anneal in the STI formation incurs problems for the formation of complementary metal-oxide-semiconductor (CMOS) image sensor (CIS) chips. The CIS chips have a strict requirement to the leakage of devices. Accordingly, in the STI formation processes that adopt HARP, after the formation of the silicon oxide liners (which are formed using in-situ steam generation (ISSG)) in the STI openings, a field implantation is performed to implant a p-type impurity into the STI openings. The p-type impurity forms P+ regions in the respective substrate regions that adjacent the STI regions. The P+ regions adjoin the silicon oxide liners. The P+ regions may help reduce the leakage of the respective image sensors such as photo diodes.
In subsequent steps, the gap filling is performed, and the STI openings are filled with a dielectric material. An anneal is then performed to condense the dielectric material. The anneal causes the p-type impurity to diffuse away from the implanted regions, and hence the leakage-prevention ability of the P+ regions is adversely affected. Furthermore, the diffusion of the p-type impurity may cause the adjacent n-wells to shrink.